The long-term objective of this research is to understand how brain cells produce specific behavioral responses. Because lordosis, a component of female reproductive behavior, is strictly dependent on the ovarian steroid hormones estradiol (E2) and progesterone (P), examination of mechanisms that control lordosis allow us to ask how hormonal signals are recognized by brain cells and translated into specific changes in behavior. The research proposed in this application combines behavioral, biochemical, and pharmacological approaches to address specific questions raised by our prior work regarding the neural mechanisms of E2 regulation of lordosis behavior in female rats. Specific Aim 1 uses in vivo microdialysis to test the hypotheses: (1) that somatosensory (flank/perineal and/or vaginocervical) rather than olfactory stimuli associated with copulation are responsible for norepinephrine (NE) release from the hypothalamus of hormone-treated, sexually receptive female rats, and (2) that steroid priming potentiates somatosensory stimulus-evoked NE release. Specific Aim 2 uses biochemical pharmacological approaches to test the hypothesis that E2 modifies opioid receptor signaling in the hypothalamus such that pathways inhibitory to lordosis (e.g., mu receptor-linked) are attenuated while pathways that facilitate lordosis (e.g., kappa and delta receptor-linked) are enhanced. Specific Aim 3 tests the hypothesis that the second messenger cGMP and its cellular effectors are hormone-regulated mediators of the facilitatory actions of E2 and P on lordosis behavior in female rats. Neuropharmacological studies will examine whether inhibition of cGMP-dependent protein kinase (PKG) reduces lordosis, and microdialysis studies will evaluate whether there is efflux of cGMP in the hypothalamus during mating tests in hormone- primed female rats. Immunological, biochemical and molecular biological methods will determine whether hormones regulate the expression of guanylyl cyclase, of PKGs, and/or of a specific PKG substrate in brain areas that regulate reproduction. These studies will provide novel insights into the cellular and molecular mechanisms by which ovarian sterioids act in the brain and to produce predictable behavioral changes.